I don't think there is a set distance,really.
Your best bet is to just stay aware of your dog,
varying heat and humidity conditions will make changes to the distance your dog may want to travel and/or be able to travel.
My male is almost 2 and there is days I can walk him 5-10 miles no problem and then there is days he has had enough after 3....
Main thing is to not set a distance but to allow your dog to pace it themselves.
Hope this helps.
It all depends on the dog a good way to gage this is to watch the dog when the dog is actually burning calories is when he begins to breath with his mouth open this is the dogs way of sweating. So would push him a little past this point until he is in a heavy but some what rhythmic pant but not gasping for air. Hope this makes since.